I've used Terragen since the first week it was offered as beta software. It's just so cool. Above is an image I created about three years ago, using a DEM from the U.S. Geological Survey of the Black Canyon of the Gunnison (Gunnison, Colo.). Terragen is fun to use. Matt Fairclough just has one of the best sky algorithms around. I can't wait until the next version accommodates arbitrary objects and has some vegitation. My favorite Terragen artist of late is Jordano here at Renderosity. Check him out. He is amazing with Terragen, and his postwork is some of the best I've ever seen.